The present invention relates in general to the control of turbo compressors and in particular to a new and useful apparatus and method of operating turbo compressors, particularly those of large capacity, comprising continuously measuring the rate of flow of the compressor, or a signal derived therefrom, and the discharge pressure or a pressure ratio of the compressor. The measured values are compared with permissible values therefor. In order to prevent surging, for example, before the reaching of a surge limit, it is insured that the rate of flow of the compressor does not fall below a minimum value which depends on the discharge pressure. This is accomplished through a closed loop control with analog and digital components, which operates to open one or more blowoff valves upon the attainment of a blowoff line which extends parallel to the surge limit on a pressure/flow characteristic curve of the compressor.
Surge limit controls which employ mechanicalhydraulic controllers are known. In spite of this very expensive equipment, however, with such prior art controls, the surge limit cannot be reproduced closely enough to reliably prevent surging. Another disadvantage of these hydraulic controls are their high maintenance and a considerable susceptibility to disturbances.
Further known is an electronic control for the surge limit, such as disclosed in the German publication "Mitteilung 542 der Warmestelle des Vereins deutscher Eisenhuttenleute" (communication 542 of The Heat Division of the Association of ferman Metallurgists). According to the disclosure, the surge limit control in a compressor with variable guide vane is similar to that designed for compressors with an adjustable throttle, with the difference, however, that a function generator is provided for forming the reference input of the surge limit controller, because of the non-linear curve of the surge limit.
It is a disadvantage in these prior art controls that at certain operating conditions, for example upon a manual intervention in the control or under strong pressure variations, surges in the compressor cannot be prevented with satisfactory reliability.
From German OS No. 26 23 899, an electronic control of the surge limit is known in which the output signal, depending on the actual values of pressure and flow, of the surge limit controller operating to adjust the blowoff valve, is amplified non-linearly, namely the amplification is augments if the error signal value is negative, i.e. if the operating pressure of the compressor passes into an extreme region, beyond the blowoff characteristic line. Further, in that disclosure, an extreme value selector connected ahead of the controller is responsive to the maximum error signal value, namely to the error signal value proper, or to the difference between the controller output and the manually set control signal.
This very satisfactorily operating control has the disadvantage, however, that it cannot follow the variation rate of the actual value, or evaluate whether the error signal value increases or decreases. In practice, this means that the blowoff line remains adjusted to a constant value, independently of the operating conditions.
A control method is also known, in which the position of the blowoff line within the characteristic is made dependent on the rate at which the working point moves through the performance graph.
It is a disadvantage in such and similar circuits, however, that the circuit elements operating in a purely analogous way become very expensive if various additional parameters, for example, the temperature, pressure, humidity at the suction side, or the molecular weight, are to be taken into account, or if the blowoff characteristic line is of a shape which is difficult to produce.
Brochures which have been distributed by the assigness of the present application disclose an analog system which measures flow rate through an output pressure from a compressor to produce a signal which is applied to a PI controller to control the position of a blowoff valve (Turbolog-Electronic Control System for GHH Turbomachinery from "Machinery News 3" published by M.A.N. Division GHH Sterkrade Oberhausen.
Such an analog control has the advantage of responding quickly to changes in flow and pressure but has the disadvantage of following a blowoff line which is at substantial distance from an optimum surge limit. This distance must be maintained since the factors noted above, such as temperature, humidity and molecular weight, are not taken into account.
Also see U.S. Pat. Nos. 4,298,310 and 4,384,818, both to Blotenberg, which disclose analog systems for controlling a blowoff valve of a turbo compressor. These two patents are incorporated by reference here.
A relationship is known which relates the socalled adiabatic head of a compressor to inlet and outlet pressure as well as variable such as inlet temperature, molecular weight and humidity. The adiabatic head is a more exact measurement of the compressor performance than outlet pressure which is generally used for convenience and as an approximation of adiabatic head.
While adiabatic head cannot be measured directly, it can be calculated using the relationship: ##EQU1## where: .DELTA.h.sub.ad =adiabatic head
K=adiabatic index PA0 R=gas constant PA0 T.sub.1 =inlet temperature PA0 P.sub.1 =inlet pressure PA0 P.sub.2 =outlet pressure.
This relationship is known from E. Truckenbrodt: Stromungsmechanik Springer Verlag Berlin Heidelberg New York 1969, page 54.
It is known that the adiabatic index is a function of molecular weight, humidity and the like. See U.S. Pat. No. 4,156,578 to Agar et al which is incorporated here by reference.
It is also generally known to utilize digital computers or microporcessors for some control functions in plants, including plants that might use a turbo compressor. Such microprocessors or computers which are available from U.S. manufacturers are the Honeywell TDC 2000; the Micon P200; the Foxboro Microspec/Spectrum, the Fisher Controls Provox and many others. Computers available from manufacturers outside the United States include the Siemens Teleperm M; the Hartmann & Braun Contronic 3; the Kent P4000 and the Yokogawa Centum.
As known in the art, these digital computers can perform various calculations. This would include the calculation identified above for calculating adiabatic head. A drawback of such computers, however, is that they have a finite cycle time. In other words, a computation must be made on the basis of measurements taken at a certain point in time. Since it takes a certain amount of time to perform the calculations and generate a signal which could be used for control, the variable may change during this interim. It is generally known that this cycle time for above mentioned systems can be about 0.2 to 0.5 seconds. This delay is not acceptable for turbo compressors anti-surge control which can be damaged if operated beyong the surge limit even for vey short periods of time.
Another problem associated with the use of digital computers or microprocessors as a control mechanism is their relatively high failure rate which is a function of their complexity.